News Bits

by Luke Muehlhauser on July 5, 2011 in News

Those who think I was exaggerating the importance of Bayes’ Rule in my tutorial might want to read the new book about its history: The Theory That Would Not Die: How Bayes’ Rule Cracked the Enigma Code, Hunted Down Russian Submarines, and Emerged Triumphant from Two Centuries of Controversy.

I wrote a new page explaining the point of

New Less Wrong post: Not for the Sake of Selfishness Alone.

I agree with The Assumptions of the Seduction Community and Pickup and Seduction Techniques for Feminists.

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{ 11 comments… read them below or add one }

Reginald Selkirk July 5, 2011 at 11:33 am

In the 1930s the USA planned pre-emptive war against UK and Canada, but then WWII got in the way so we became allies.

Concerns in some quarters notwithstanding, the whole thing was just a theoretical exercise in military planning.

The U.S. military makes plans for every contingency they can think of. That is not the same as saying that anyone in power intended to implement those plans. You are misleading your readers by sloppy use of the word “planned.”


Reginald Selkirk July 5, 2011 at 11:39 am

Oh, and guess what? the U.S. military still has plans to invade Canada.

And how would Canada defend itself in the event of an invasion?

“We’ve got thousands of Canadians in the U.S. right now, in place secretly,” he said. “They could be on your street. We’ve sent people like Celine Dion and Mike Myers to secretly infiltrate American society.”


John D July 5, 2011 at 12:21 pm

That book on Bayes’ Rule looks pretty interesting. Thanks for the pointer


Bill Maher July 5, 2011 at 1:37 pm

that breaks my heart that Cars 2 sucks. PIXAR has always delivered gold. I have said “they can not make another movie this good” like five times about their films.


epistememe July 5, 2011 at 3:48 pm

I thought this review of the book was rather insightful.

“Sharon Bertsch Mcgrayne is a talented science writer whose portraits of great scientists of the past are incisive and entertaining. However, she evidently believes that one must studiously avoid dealing with any serious scientific issues in entertaining a popular audience. For this reason, this book is a total failure. Why should a reader care about the history of an idea of which he or she has zero understanding? Mcgrayne turns the history of Bayes rule into a pitched battle between intransigent opponents, but we never find out what the real issue are.

In fact, Bayes rule is a mathematical tautology, being the definition of conditional probability. Suppose A is an event with probability P(A) and B is an event with probability P(B). Let C be the event “both A and B occur.” Then the conditional probability P(A|B) of event A, given that we know that B has occurred, just P(C)/P(B). Moreover, if a decision-maker knows P(A), P(B), and P(C), and discovers that B occurred, then he should revise the probability that A occurred to P(A|B) = P(C)/P(B). Why? Well, suppose we have a population of 1000 individuals, where the probability that an event E is true of an individual is P(E), where E is any one of A, B, and C. Then the expected number of individuals for which B is true is 1000*P(B). Of these, the number for which A is also true is 1000*P(C). Therefore, the probability that an individual satisfies A, given that he satisfies B, is 1000*P(C)/1000*P(B) = P(A|B).

For instance, suppose 5% of the population uses drugs, and there is a drug test that is 95% accurate: it
tests positive on a drug user 95% of the time, and it tests negative on a drug nonuser 95% of the time. If an individual tests positive, we can show using Bayes rule that the probability of his being a drug user is 50%. To see this, let A be the event “subject uses drugs,” and let B be the event “subject test positive for using drugs.” First, what is the probability P(B) of event B? Well, take a random subject. With probability 1/20 he is a drug user, so with probability (19/20)(1/20)=19/400 he is a drug user testing positive. With probability 19/20 he not a drug user, so he is a non-user testing positive with probability (1/20)(19/20)=19/400. Thus P(B) = 19/400+19/400=38/400. Let event C be “subject uses drugs and tests positive for using drugs.” This probability is (1/20) times (19/20) = 19/400. Thus P(A|B) = P(C)/P(B) = 1/2.

If this seems mystifying, consider the following interpretation. Suppose we test 10000 people. The expected number of drug users will be 500, and 95% of them, or 475, will test positive for drug use. But 9500 people will be non-drug users, and 5% of them will erroneously test positive for drug use, which is 475 people. Thus, 50% of those who test positive for drugs are actually drug users.

Now, who could dispute this analysis? It is clearly correct. So where does all of the vehement opposition to Bayes rule come from? The answer is that when a group of individuals (e.g., professional scientists) do not agree on P(A) then you cannot apply Bayes rule. You can however show that under many conditions, repeated observations of events B can lead to mutually acceptable values for P(A). For instance, suppose you know that the weight of a substance per ounce is variable and unknown, and each scientist i has his personal prior probability Pi that the weight is less than one gram per ounce. Suppose we take unbiased samples that are each about one ounce, and we take unbiased measurements of the weight. Then the long-run average of the sample weights will be accepted by all scientists as the updated probability. This is Bayesian updating.

However, it is not true that Bayesian updating always lead to convergence to a common probability distribution. See, for instance, papers by Mordecai Kurz, of Stanford University. Moreover, when observations are limited, the range of assessments of probabilities can be quite wide. This is why Bayes rule is considered “subjective.” However, when we really know the probabilities, as in the case of the drug testing example, there is no controversy about the value of Bayes rule. It is extremely valuable, indeed indispensable, in such cases.

This book manages to obfuscate a very simple issue, turning sciences into a vast morality play. Now of course there are deep issues in the philosophy of probability that implicate Bayes rule, but one does not learn what they are from this book.”


Justfinethanks July 6, 2011 at 10:22 am

Philosophical comedy: Children force you into deep existential and metaphysical quandries.

My kid makes me feel stupid all the time. Once when we were watching The Sorcerer’s Apprentice segment from Fantasia, my daughter said:

Her: “Who’s that?”
Me: “That’s the sorcerer.”
Her: “What’s a sorcerer?”
Me: “Someone who does magic.”
Her: “What’s magic?”
Me: “I guess it would be… the supernatural manipulation of reality.”
Her: “What’s reality?”
Me: “Uh… well, whatever is the truth about about the world we live in, that’s reality.”
Her: “What’s truth?”
Me: “I don’t know, Pontius. Just watch the damn movie.”


Bill Maher July 6, 2011 at 12:25 pm


my daughter does the same thing.


Tarun July 6, 2011 at 2:25 pm


That review seems to conflate Bayes’ theorem with Bayesian updating. Bayes’ theorem is certainly a tautology, but it is a claim about a single probability distribution. Bayesian updating, which is at the center of the Bayesian/frequentist quarrel, is a claim about how to change one’s probability distribution upon acquiring new evidence. It is most certainly not tautologous. It’s most common justification (the diachronic Dutch book argument) is a pragmatic argument, not a mathematical demonstration.

There are, in fact, situations where Bayesian updating does not seem like the correct rule to follow. Examples are cases where the Reflection Principle fails, much discussed in the recent philosophical literature.


Dreaded Anomaly July 8, 2011 at 12:37 am


I would be interested in your take on this recent Richard Dawkins vs. Rebecca Watson et al conflict.

Personally, I generally agree with Rebecca’s original point (which seems to me to be primarily about comfort level, as is also mentioned in the “techniques for feminists” link). I think that Dawkins missed that point originally, and then a confluence of stubborn personalities and preexisting anger led to unnecessary escalation.


Luke Muehlhauser July 8, 2011 at 7:50 am

Dreaded Anomaly,

I probably agree most with Gawker’s points. :)


Reginald Selkirk July 10, 2011 at 10:46 am

Tool use by fish documented

yet another blow to human exceptionalism


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